Methods of Modifying Eukaryotic Cells

ABSTRACT

A method for engineering and utilizing large DNA vectors to target, via homologous recombination, and modify, in any desirable fashion, endogenous genes and chromosomal loci in eukaryotic cells. These large DNA targeting vectors for eukaryotic cells, termed LTVECs, are derived from fragments of cloned genomic DNA larger than those typically used by other approaches intended to perform homologous targeting in eukaryotic cells. Also provided is a rapid and convenient method of detecting eukaryotic cells in which the LTVEC has correctly targeted and modified the desired endogenous gene(s) or chromosomal locus (loci) as well as the use of these cells to generate organisms bearing the genetic modification.

This application is a continuation of U.S. patent application Ser. No.11/809,473, filed 1 Jun. 2007, which is a continuation of U.S. patentapplication Ser. No. 10/415,440, filed 29 Apr. 2003, which is NationalStage of International Application No. PCT/US01/45375, filed 31 Oct.2001 and published in English under PCT Article 21(2), which claims thebenefit of U.S. patent application Ser. No. 09/732,234, filed 7 Dec.2000, which is now U.S. Pat. No. 6,586,251, which claims the benefit ofU.S. Provisional Application No. 60/244,665, filed 31 Oct. 2000, whichapplications are herein incorporated by reference. Throughout thisapplication various publications are referenced. The disclosures ofthese publications in their entireties are hereby incorporated byreference into this application.

FIELD OF THE INVENTION

The field of this invention is a method for engineering and utilizinglarge DNA vectors to target, via homologous recombination, and modify,in any desirable fashion, endogenous genes and chromosomal loci ineukaryotic cells. These large DNA targeting vectors for eukaryoticcells, termed LTVECs, are derived from fragments of cloned genomic DNAlarger than those typically used by other approaches intended to performhomologous targeting in eukaryotic cells. The field of the inventionfurther provides for a rapid and convenient method of detectingeukaryotic cells in which the LTVEC has correctly targeted and modifiedthe desired endogenous gene(s) or chromosomal locus (loci). The fieldalso encompasses the use of these cells to generate organisms bearingthe genetic modification, the organisms, themselves, and methods of usethereof.

INTRODUCTION

The use of LTVECs provides substantial advantages over current methods.For example, since these are derived from DNA fragments larger thanthose currently used to generate targeting vectors, LTVECs can be morerapidly and conveniently generated from available libraries of largegenomic DNA fragments (such as BAC and PAC libraries) than targetingvectors made using current technologies. In addition, largermodifications as well as modifications spanning larger genomic regionscan be more conveniently generated than using current technologies.

Furthermore, the present invention takes advantage of long regions ofhomology to increase the targeting frequency of “hard to target” loci,and also diminishes the benefit, if any, of using isogenic DNA in thesetargeting vectors.

The present invention thus provides for a rapid, convenient, andstreamlined method for systematically modifying virtually all theendogenous genes and chromosomal loci of a given organism.

BACKGROUND OF THE INVENTION

Gene targeting by means of homologous recombination between homologousexogenous DNA and endogenous chromosomal sequences has proven to be anextremely valuable way to create deletions, insertions, designmutations, correct gene mutations, introduce transgenes, or make othergenetic modifications in mice. Current methods involve using standardtargeting vectors, with regions of homology to endogenous DNA typicallytotaling less than 10-20 kb, to introduce the desired geneticmodification into mouse embryonic stem (ES) cells, followed by theinjection of the altered ES cells into mouse embryos to transmit theseengineered genetic modifications into the mouse germline (Smithies etal., Nature, 317:230-234, 1985; Thomas et al., Cell, 51:503-512, 1987;Koller et al., Proc Natl Acad Sci USA, 86:8927-8931, 1989; Kuhn et al.,Science, 254:707-710, 1991; Thomas et al., Nature, 346:847-850, 1990;Schwartzberg et al., Science, 246:799-803, 1989; Doetschman et al.,Nature, 330:576-578, 1987; Thomson et al., Cell, 5:313-321, 1989;DeChiara et al., Nature, 345:78-80, 1990; U.S. Pat. No. 5,789,215,issued Aug. 4, 1998 in the name of GenPharm International) In thesecurrent methods, detecting the rare ES cells in which the standardtargeting vectors have correctly targeted and modified the desiredendogenous gene(s) or chromosomal locus(loci) requires sequenceinformation outside of the homologous targeting sequences containedwithin the targeting vector. Assays for successful targeting involvestandard Southern blotting or long PCR (Cheng, et al., Nature,369:684-5, 1994; Foord and Rose, PCR Methods Appl, 3:S149-61, 1994;Ponce and Micol, Nucleic Acids Res, 20:623, 1992; U.S. Pat. No.5,436,149 issued to Takara Shuzo Co., Ltd.) from sequences outside thetargeting vector and spanning an entire homology arm (see Definitions);thus, because of size considerations that limit these methods, the sizeof the homology arms are restricted to less than 10-20 kb in total(Joyner, The Practical Approach Series, 293, 1999).

The ability to utilize targeting vectors with homology arms larger thanthose used in current methods would be extremely valuable. For example,such targeting vectors could be more rapidly and conveniently generatedfrom available libraries containing large genomic inserts (e.g. BAC orPAC libraries) than targeting vectors made using current technologies,in which such genomic inserts have to be extensively characterized andtrimmed prior to use. In addition, larger modifications as well asmodifications spanning larger genomic regions could be more convenientlygenerated and in fewer steps than using current technologies.Furthermore, the use of long regions of homology could increase thetargeting frequency of “hard to target” loci in eukaryotic cells, sincethe targeting of homologous recombination in eukaryotic cells appears tobe related to the total homology contained within the targeting vector(Deng and Capecchi, Mol Cell Biol, 12:3365-71, 1992). In addition, theincreased targeting frequency obtained using long homology arms coulddiminish any potential benefit that can be derived from using isogenicDNA in these targeting vectors.

The problem of engineering precise modifications into very large genomicfragments, such as those cloned in BAC libraries, has largely beensolved through the use of homologous recombination in bacteria (Zhang,et al., Nat Genet, 20:123-8, 1998; Yang, et al., Nat Biotechnol,15:859-65, 1997; Angrand, et al., Nucleic Acids Res, 27:e16, 1999;Muyrers, et al., Nucleic Acids Res, 27:1555-7, 1999; Narayanan, et al.,Gene Ther, 6:442-7, 1999), allowing for the construction of vectorscontaining large regions of homology to eukaryotic endogenous genes orchromosomal loci. However, once made, these vectors have not beengenerally useful for modifying endogenous genes or chromosomal loci viahomologous recombination because of the difficulty in detecting rarecorrect targeting events when homology arms are larger than 10-20 kb(Joyner, The Practical Approach Series, 293, 1999). Consequently,vectors generated using bacterial homologous recombination from BACgenomic fragments must still be extensively trimmed prior to use astargeting vectors (Hill et al., Genomics, 64:111-3, 2000). Therefore,there is still a need for a rapid and convenient methodology that makespossible the use of targeting vectors containing large regions ofhomology so as to modify endogenous genes or chromosomal loci ineukaryotic cells.

In accordance with the present invention, Applicants provide novelmethods that enable the use of targeting vectors containing largeregions of homology so as to modify endogenous genes or chromosomal lociin eukaryotic cells via homologous recombination. Such methods overcomethe above-described limitations of current technologies. In addition,the skilled artisan will readily recognize that the methods of theinvention are easily adapted for use with any genomic DNA of anyeukaryotic organism including, but not limited to, animals such asmouse, rat, other rodent, or human, as well as plants such as soy, cornand wheat.

SUMMARY OF THE INVENTION

In accordance with the present invention, Applicants have developed anovel, rapid, streamlined, and efficient method for creating andscreening eukaryotic cells, which contain modified endogenous genes orchromosomal loci. This novel methods combine, for the first time:

-   -   1. Bacterial homologous recombination to precisely engineer a        desired genetic modification within a large cloned genomic        fragment, thereby creating a large targeting vector for use in        eukaryotic cells (LTVECs);    -   2. Direct introduction of these LTVECs into eukaryotic cells to        modify the endogenous chromosomal locus of interest in these        cells; and    -   3. An analysis to determine the rare eukaryotic cells in which        the targeted allele has been modified as desired, involving an        assay for modification of allele (MOA) of the parental allele        that does not require sequence information outside of the        targeting sequence, such as, for example, quantitative PCR.

A preferred embodiment of the invention is a method for geneticallymodifying an endogenous gene or chromosomal locus in eukaryotic cells,comprising: a) obtaining a large cloned genomic fragment containing aDNA sequence of interest; b) using bacterial homologous recombination togenetically modify the large cloned genomic fragment of (a) to create alarge targeting vector for use in the eukaryotic cells (LTVEC); c)introducing the LTVEC of (b) into the eukaryotic cells to modify theendogenous gene or chromosomal locus in the cells; and d) using aquantitative assay to detect modification of allele (MOA) in theeukaryotic cells of (c) to identify those eukaryotic cells in which theendogenous gene or chromosomal locus has been genetically modified.

Another embodiment of the invention is a method wherein the geneticmodification to the endogenous gene or chromosomal locus comprisesdeletion of a coding sequence, gene segment, or regulatory element;alteration of a coding sequence, gene segment, or regulatory element;insertion of a new coding sequence, gene segment, or regulatory element;creation of a conditional allele; or replacement of a coding sequence orgene segment from one species with an homologous or orthologous codingsequence from a different species.

An alternative embodiment of the invention is a method wherein thealteration of a coding sequence, gene segment, or regulatory elementcomprises a substitution, addition, or fusion, wherein the fusioncomprises an epitope tag or bifunctional protein.

Yet another embodiment of the invention is a method wherein thequantitative assay comprises quantitative PCR, comparative genomichybridization, isothermal DNA amplification, quantitative hybridizationto an immobilized probe, Invader Probes®, or MMP assays®, and whereinthe quantitative PCR comprises TaqMan® Molecular Beacon, or Eclipse™probe technology.

Another preferred embodiment of the invention is a method wherein theeukaryotic cell is a mammalian embryonic stem cell and in particularwherein the embryonic stem cell is a mouse, rat, or other rodentembryonic stem cell.

Another preferred embodiment of the invention is a method wherein theendogenous gene or chromosomal locus is a mammalian gene or chromosomallocus, preferably a human gene or chromosomal locus or a mouse, rat, orother rodent gene or chromosomal locus.

An additional preferred embodiment is one in which the LTVEC is capableof accommodating large DNA fragments greater than 20 kb, and inparticular large DNA fragments greater than 100 kb.

Another preferred embodiment is a genetically modified endogenous geneor chromosomal locus that is produced by the method of the invention.

Yet another preferred embodiment is a genetically modified eukaryoticcell that is produced by the method of the invention.

A preferred embodiment of the invention is a non-human organismcontaining the genetically modified endogenous gene or chromosomal locusproduced by the method of the invention.

Also preferred in a non-human organism produced from the geneticallymodified eukaryotic cells or embryonic stem cells produced by the methodof the invention.

A preferred embodiment is a non-human organism containing a geneticallymodified endogenous gene or chromosomal locus, produced by a methodcomprising the steps of: a) obtaining a large cloned genomic fragmentcontaining a DNA sequence of interest; b) using bacterial homologousrecombination to genetically modify the large cloned genomic fragment of(a) to create a large targeting vector (LTVEC) for use in embryonic stemcells; c) introducing the LTVEC of (b) into the embryonic stem cells tomodify the endogenous gene or chromosomal locus in the cells; d) using aquantitative assay to detect modification of allele (MOA) in theembryonic stem cells of (c) to identify those embryonic stem cells inwhich the endogenous gene or chromosomal locus has been geneticallymodified; e) introducing the embryonic stem cell of (d) into ablastocyst; and f) introducing the blastocyst of (e) into a surrogatemother for gestation.

An additional preferred embodiment of the invention is a non-humanorganism containing a genetically modified endogenous gene orchromosomal locus, produced by a method comprising the steps of: a)obtaining a large cloned genomic fragment containing a DNA sequence ofinterest; b) using bacterial homologous recombination to geneticallymodify the large cloned genomic fragment of (a) to create a largetargeting vector for use in eukaryotic cells (LTVEC); c) introducing theLTVEC of (b) into the eukaryotic cells to genetically modify theendogenous gene or chromosomal locus in the cells; d) using aquantitative assay to detect modification of allele (MOA) in theeukaryotic cells of (c) to identify those eukaryotic cells in which theendogenous gene or chromosomal locus has been genetically modified; e)removing the nucleus from the eukaryotic cell of (d); f) introducing thenucleus of (e) into an oocyte; and g) introducing the oocyte of (f) intoa surrogate mother for gestation.

Yet another preferred embodiment is a non-human organism containing agenetically modified endogenous gene or chromosomal locus, produced by amethod comprising the steps of: a) obtaining a large cloned genomicfragment containing a DNA sequence of interest; b) using bacterialhomologous recombination to genetically modify the large cloned genomicfragment of (a) to create a large targeting vector for use in eukaryoticcells (LTVEC); c) introducing the LTVEC of (b) into the eukaryotic cellsto genetically modify the endogenous gene or chromosomal locus in thecells; d) using a quantitative assay to detect modification of allele(MOA) in the eukaryotic cells of (c) to identify those eukaryotic cellsin which the endogenous gene or chromosomal locus has been geneticallymodified; e) fusing the eukaryotic cell of (d) with another eukaryoticcell; f) introducing the fused eukaryotic cell of (e) into a surrogatemother for gestation.

In preferred embodiments, the non-human organism is a mouse, rat, orother rodent; the blastocyst is a mouse, rat, or other rodentblastocyst; the oocyte is a mouse, rat, or other rodent oocyte; and thesurrogate mother is a mouse, rat, or other rodent.

Another preferred embodiment is one in which the embryonic stem cell isa mammalian embryonic stem cell, preferably a mouse, rat, or otherrodent embryonic stem cell.

An additional preferred embodiment is the use of the geneticallymodified eukaryotic cells of the invention for the production of anon-human organism, and in particular, the use of the geneticallymodified embryonic stem cell of the invention for the production of anon-human organism.

A preferred embodiment of the invention is a method for geneticallymodifying an endogenous gene or chromosomal locus of interest in mouseembryonic stem cells, comprising: a) obtaining a large cloned genomicfragment greater than 20 kb which contains a DNA sequence of interest,wherein the large cloned DNA fragment is homologous to the endogenousgene or chromosomal locus; b) using bacterial homologous recombinationto genetically modify the large cloned genomic fragment of (a) to createa large targeting vector for use in the mouse embryonic stem cells,wherein the genetic modification is deletion of a coding sequence, genesegment, or regulatory element; c) introducing the large targetingvector of (b) into the mouse embryonic stem cells to modify theendogenous gene or chromosomal locus in the cells; and d) using aquantitative assay to detect modification of allele (MOA) in the mouseembryonic stem cells of (c) to identify those mouse embryonic stem cellsin which the endogenous gene or chromosomal locus has been geneticallymodified, wherein the quantitative assay is quantitative PCR. Alsopreferred is a genetically modified mouse embryonic stem cell producedby this method; a mouse containing a genetically modified endogenousgene or chromosomal locus produced by this method; and a mouse producedfrom the genetically modified mouse embryonic stem cell.

Another preferred embodiment is a mouse containing a geneticallymodified endogenous gene or chromosomal locus of interest, produced by amethod comprising the steps of: a) obtaining a large cloned genomicfragment greater than 20 kb which contains a DNA sequence of interest,wherein the large cloned DNA fragment is homologous to the endogenousgene or chromosomal locus; b) using bacterial homologous recombinationto genetically modify the large cloned genomic fragment of (a) to createa large targeting vector for use in the mouse embryonic stem cells,wherein the genetic modification is deletion of a coding sequence, genesegment, or regulatory element; c) introducing the large targetingvector of (b) into the mouse embryonic stem cells to modify theendogenous gene or chromosomal locus in the cells; and d) using aquantitative assay to detect modification of allele (MOA) in the mouseembryonic stem cells of (c) to identify those mouse embryonic stem cellsin which the endogenous gene or chromosomal locus has been geneticallymodified, wherein the quantitative assay is quantitative PCR; e)introducing the mouse embryonic stem cell of (d) into a blastocyst; andf) introducing the blastocyst of (e) into a surrogate mother forgestation.

Also preferred is the use of the genetically modified mouse embryonicstem cell described above for the production of a mouse.

Also preferred are methods wherein 1-5 μg of large targeting vector DNAis introduced into 1×10⁷ eukaryotic cells.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Schematic diagram of the generation of a typical LTVEC usingbacterial homologous recombination. (hb1=homology box 1; hb2=homologybox 2; RE=restriction enzyme site).

FIG. 2: Schematic diagram of donor fragment and LTVEC for mouse OCR10.(hb1=homology box 1; lacZ=β-galactosidase ORF; SV40 polyA=a DNA fragmentderived from Simian Virus 40, containing a polyadenylation site andsignal; PGKp=mouse phosphoglycerate kinase (PGK) promoter; EM7=abacterial promoter; neo=neomycin phosphotransferase; PGK polyA=3′untranslated region derived from the PGK gene and containing apolyadenylation site and signal; hb2=homology box 2)

FIG. 3A-3D: Sequence of the mouse OCR10 cDNA, homology box 1 (hb1),homology box 2 (hb2), and TaqMan® probes and primers used in aquantitative PCR assay to detect modification of allele (MOA) in EScells targeted using the mOCR10 LTVEC.

hb1: base pairs 1 to 211

hb2: base pairs 1586 to 1801

TaqMan® probe and corresponding PCR primer set derived from mOCR10 exon3:

TaqMan® probe: nucleotides 413 to 439—upper strand

Primer ex3-5′: nucleotides 390 to 410—upper strand

Primer ex3-3′: nucleotides 445 to 461—lower strand

TaqMan® probe and corresponding PCR primer set derived from mOCR10 exon4:

TaqMan® probe: nucleotides 608 to 639—upper strand

Primer ex4-5′: nucleotides 586 to 605—upper strand

Primer ex4-3′: nucleotides 642 to 662—lower strand

DEFINITIONS

A “targeting vector” is a DNA construct that contains sequences“homologous” to endogenous chromosomal nucleic acid sequences flanking adesired genetic modification(s). The flanking homology sequences,referred to as “homology arms”, direct the targeting vector to aspecific chromosomal location within the genome by virtue of thehomology that exists between the homology arms and the correspondingendogenous sequence and introduce the desired genetic modification by aprocess referred to as “homologous recombination”.

“Homologous” means two or more nucleic acid sequences that are eitheridentical or similar enough that they are able to hybridize to eachother or undergo intermolecular exchange.

“Gene targeting” is the modification of an endogenous chromosomal locusby the insertion into, deletion of, or replacement of the endogenoussequence via homologous recombination using a targeting vector.

A “gene knockout” is a genetic modification resulting from thedisruption of the genetic information encoded in a chromosomal locus.

A “gene knockin” is a genetic modification resulting from thereplacement of the genetic information encoded in a chromosomal locuswith a different DNA sequence.

A “knockout organism” is an organism in which a significant proportionof the organism's cells harbor a gene knockout.

A “knockin organism” is an organism in which a significant proportion ofthe organism's cells harbor a gene knockin.

A “marker” or a “selectable marker” is a selection marker that allowsfor the isolation of rare transfected cells expressing the marker fromthe majority of treated cells in the population. Such marker's gene'sinclude, but are not limited to, neomycin phosphotransferase andhygromycin B phosphotransferase, or fluorescing proteins such as GFP.

An “ES cell” is an embryonic stem cell. This cell is usually derivedfrom the inner cell mass of a blastocyst-stage embryo.

An “ES cell clone” is a subpopulation of cells derived from a singlecell of the ES cell population following introduction of DNA andsubsequent selection.

A “flanking DNA” is a segment of DNA that is collinear with and adjacentto a particular point of reference.

“LTVECs” are large targeting vectors for eukaryotic cells that arederived from fragments of cloned genomic DNA larger than those typicallyused by other approaches intended to perform homologous targeting ineukaryotic cells.

A “non-human organism” is an organism that is not normally accepted bythe public as being human.

“Modification of allele” (MOA) refers to the modification of the exactDNA sequence of one allele of a gene(s) or chromosomal locus (loci) in agenome. This modification of allele (MOA) includes, but is not limitedto, deletions, substitutions, or insertions of as little as a singlenucleotide or deletions of many kilobases spanning a gene(s) orchromosomal locus (loci) of interest, as well as any and all possiblemodifications between these two extremes. “Orthologous” sequence refersto a sequence from one species that is the functional equivalent of thatsequence in another species.

The description and examples presented infra are provided to illustratethe subject invention. One of skill in the art will recognize that theseexamples are provided by way of illustration only and are not includedfor the purpose of limiting the invention.

DETAILED DESCRIPTION OF THE INVENTION

Applicants have developed a novel, rapid, streamlined, and efficientmethod for creating and screening eukaryotic cells, which containmodified endogenous genes or chromosomal loci. In these cells, themodification may be gene(s) knockouts, knockins, point mutations, orlarge genomic insertions or deletions or other modifications. By way ofnon-limiting example, these cells may be embryonic stem cells, which areuseful for creating knockout or knockin organisms, and in particular,knockout or knockin mice, for the purpose of determining the function ofthe gene(s) that have been altered, deleted and/or inserted.

The novel methods described herein combine, for the first time:

-   -   1. Bacterial homologous recombination to precisely engineer a        desired genetic modification within a large cloned genomic DNA        fragment, thereby creating a large targeting vector for use in        eukaryotic cells (LTVECs);    -   2. Direct introduction of these LTVECs into eukaryotic cells to        modify the corresponding endogenous gene(s) or chromosomal locus        (loci) of interest in these cells; and    -   3. An analysis to determine the rare eukaryotic cells in which        the targeted allele has been modified as desired, involving a        quantitative assay for modification of allele (MOA) of the        parental allele.

It should be emphasized that previous methods to detect successfulhomologous recombination in eukaryotic cells cannot be utilized inconjunction with the LTVECs of Applicants' invention because of the longhomology arms present in the LTVECs. Utilizing a LTVEC to deliberatelymodify endogenous genes or chromosomal loci in eukaryotic cells viahomologous recombination is made possible by the novel application of anassay to determine the rare eukaryotic cells in which the targetedallele has been modified as desired, such assay involving a quantitativeassay for modification of allele (MOA) of a parental allele, byemploying, for example, quantitative PCR or other suitable quantitativeassays for MOA.

The ability to utilize targeting vectors with homology arms larger thanthose used in current methods is extremely valuable for the followingreasons:

-   -   1. Targeting vectors are more rapidly and conveniently generated        from available libraries containing large genomic inserts (e.g.        BAC or PAC libraries) than targeting vectors made using previous        technologies, in which the genomic inserts have to be        extensively characterized and “trimmed” prior to use (explained        in detail below). In addition, minimal sequence information        needs to be known about the locus of interest, i.e. it is only        necessary to know the approximately 80-100 nucleotides that are        required to generate the homology boxes (described in detail        below) and to generate probes that can be used in quantitative        assays for MOA (described in detail below).    -   2. Larger modifications as well as modifications spanning larger        genomic regions are more conveniently generated and in fewer        steps than using previous technologies. For example, the method        of the invention makes possible the precise modification of        large loci that cannot be accommodated by traditional        plasmid-based targeting vectors because of their size        limitations. It also makes possible the modification of any        given locus at multiple points (e.g. the introduction of        specific mutations at different exons of a multi-exon gene) in        one step, alleviating the need to engineer multiple targeting        vectors and to perform multiple rounds of targeting and        screening for homologous recombination in ES cells.    -   3. The use of long regions of homology (long homology arms)        increase the targeting frequency of “hard to target” loci in        eukaryotic cells, consistent with previous findings that        targeting of homologous recombination in eukaryotic cells        appears to be related to the total homology contained within the        targeting vector.    -   4. The increased targeting frequency obtained using long        homology arms apparently diminishes the benefit, if any, from        using isogenic DNA in these targeting vectors.    -   5. The application of quantitative MOA assays for screening        eukaryotic cells for homologous recombination not only empowers        the use of LTVECs as targeting vectors (advantages outlined        above) but also reduces the time for identifying correctly        modified eukaryotic cells from the typical several days to a few        hours. In addition, the application of quantitative MOA does not        require the use of probes located outside the endogenous gene(s)        or chromosomal locus (loci) that is being modified, thus        obviating the need to know the sequence flanking the modified        gene(s) or locus (loci). This is a significant improvement in        the way the screening has been performed in the past and makes        it a much less labor-intensive and much more cost-effective        approach to screening for homologous recombination events in        eukaryotic cells.

Methods

Many of the techniques used to construct DNA vectors described hereinare standard molecular biology techniques well known to the skilledartisan (see e.g., Sambrook, J., E. F. Fritsch And T. Maniatis.Molecular Cloning: A Laboratory Manual, Second Edition, Vols 1, 2, and3, 1989; Current Protocols in Molecular Biology, Eds. Ausubel et al.,Greene Publ. Assoc., Wiley Interscience, NY). All DNA sequencing is doneby standard techniques using an ABI 373A DNA sequencer and Taq DideoxyTerminator Cycle Sequencing Kit (Applied Biosystems, Inc., Foster City,Calif.).

Step 1. Obtain a Large Genomic DNA Clone Containing the Gene(s) orChromosomal Locus (Loci) of Interest.

A Gene(s) or locus (loci) of interest can be selected based on specificcriteria, such as detailed structural or functional data, or it can beselected in the absence of such detailed information as potential genesor gene fragments become predicted through the efforts of the variousgenome-sequencing projects. Importantly, it should be noted that it isnot necessary to know the complete sequence and gene structure of agene(s) of interest to apply the method of the subject invention toproduce LTVECs. In fact, the only sequence information that is requiredis approximately 80-100 nucleotides so as to obtain the genomic clone ofinterest as well as to generate the homology boxes used in making theLTVEC (described in detail below) and to make probes for use inquantitative MOA assays.

Once a gene(s) or locus (loci) of interest has been selected, a largegenomic clone(s) containing this gene(s) or locus (loci) is obtained.This clone(s) can be obtained in any one of several ways including, butnot limited to, screening suitable DNA libraries (e.g. BAC, PAC, YAC, orcosmid) by standard hybridization or PCR techniques, or by any othermethods familiar to the skilled artisan.

Step 2. Append Homology Boxes 1 and 2 to a Modification Cassette andGeneration of LTVEC.

Homology boxes mark the sites of bacterial homologous recombination thatare used to generate LTVECs from large cloned genomic fragments (FIG.1). Homology boxes are short segments of DNA, generally double-strandedand at least 40 nucleotides in length, that are homologous to regionswithin the large cloned genomic fragment flanking the “region to bemodified”. The homology boxes are appended to the modification cassette,so that following homologous recombination in bacteria, the modificationcassette replaces the region to be modified (FIG. 1). The technique ofcreating a targeting vector using bacterial homologous recombination canbe performed in a variety of systems (Yang et al., Nat Biotechnol,15:859-65, 1997; Muyrers et al., Nucleic Acids Res, 27:1555-7, 1999;Angrand et al., Nucleic Acids Res, 27:e16, 1999; Narayanan et al., GeneTher, 6:442-7, 1999; Yu, et al., Proc Natl Acad Sci USA, 97:5978-83,2000). One example of a favored technology currently in use is ETcloning (Zhang et al., Nat Genet, 20:123-8, 1998; Narayanan et al., GeneTher, 6:442-7, 1999) and variations of this technology (Yu, et al., ProcNatl Acad Sci USA, 97:5978-83, 2000). ET refers to the recE (Hall andKolodner, Proc Natl Acad Sci USA, 91:3205-9, 1994) and recT proteins(Kusano et al., Gene, 138:17-25, 1994) that carry out the homologousrecombination reaction. RecE is an exonuclease that trims one strand oflinear double-stranded DNA (essentially the donor DNA fragment describedinfra) 5′ to 3′, thus leaving behind a linear double-stranded fragmentwith a 3′ single-stranded overhang. This single-stranded overhang iscoated by recT protein, which has single-stranded DNA (ssDNA) bindingactivity (Kovall and Matthews, Science, 277:1824-7, 1997). ET cloning isperformed using E. coli that transiently express the E. coli geneproducts of recE and recT (Hall and Kolodner, Proc Natl Acad Sci USA,91:3205-9, 1994; Clark et al., Cold Spring Harb Symp Quant Biol,49:453-62, 1984; Noirot and Kolodner, J Biol Chem, 273:12274-80, 1998;Thresher et al., J Mol Biol, 254:364-71, 1995; Kolodner et al., MolMicrobiol, 11:23-30, 1994; Hall et al., J Bacteriol, 175:277-87, 1993)and the bacteriophage lambda (λ) protein λgam (Murphy, J Bacteriol,173:5808-21, 1991; Poteete et al., J Bacteriol, 170:2012-21, 1988). Theλgam protein is required for protecting the donor DNA fragment fromdegradation by the recBC exonuclease system (Myers and Stahl, Annu RevGenet, 28:49-70, 1994) and it is required for efficient ET-cloning inrecBC⁺ hosts such as the frequently used E. coli strain DH10b.

The region to be modified and replaced using bacterial homologousrecombination can range from zero nucleotides in length (creating aninsertion into the original locus) to many tens of kilobases (creating adeletion and/or a replacement of the original locus). Depending on themodification cassette, the modification can result in the following:

(a) deletion of coding sequences, gene segments, or regulatory elements;(b) alteration(s) of coding sequence, gene segments, or regulatoryelements including substitutions, additions, and fusions (e.g. epitopetags or creation of bifunctional proteins such as those with GFP);(c) insertion of new coding regions, gene segments, or regulatoryelements, such as those for selectable marker genes or reporter genes orputting new genes under endogenous transcriptional control;(d) creation of conditional alleles, e.g. by introduction of loxP sitesflanking the region to be excised by Cre recombinase (Abremski andHoess, J Biol Chem, 259:1509-14, 1984), or FRT sites flanking the regionto be excised by Flp recombinase (Andrews et al., Cell, 40:795-803,1985; Meyer-Leon et al., Cold Spring Harb Symp Quant Biol, 49:797-804,1984; Cox, Proc Natl Acad Sci USA, 80:4223-7, 1983); or(e) replacement of coding sequences or gene segments from one specieswith orthologous coding sequences from a different species, e.g.replacing a murine genetic locus with the orthologous human geneticlocus to engineer a mouse where that particular locus has been‘humanized’.

Any or all of these modifications can be incorporated into a LTVEC. Aspecific, non-limiting example in which an endogenous coding sequence isentirely deleted and simultaneously replaced with both a reporter geneas well as a selectable marker is provided below in Example 1, as arethe advantages of the method of the invention as compared to previoustechnologies.

Step 3 (Optional). Verify that Each LTVEC has been Engineered Correctly.

Verify that each LTVEC has been engineered correctly by:

a. Diagnostic PCR to verify the novel junctions created by theintroduction of the donor fragment into the gene(s) or chromosomal locus(loci) of interest. The PCR fragments thus obtained can be sequenced tofurther verify the novel junctions created by the introduction of thedonor fragment into the gene(s) or chromosomal locus (loci) of interest.b. Diagnostic restriction enzyme digestion to make sure that only thedesired modifications have been introduced into the LTVEC during thebacterial homologous recombination process.c. Direct sequencing of the LTVEC, particularly the regions spanning thesite of the modification to verify the novel junctions created by theintroduction of the donor fragment into the gene(s) or chromosomal locus(loci) of interest.Step 4. Purification, Preparation, and Linearization of LTVEC DNA forIntroduction into Eukaryotic Cells.a. Preparation of LTVEC DNA:

Prepare miniprep DNA (Sambrook, J., E. F. Fritsch And T. Maniatis.Molecular Cloning: A Laboratory Manual, Second Edition, Vols 1, 2, and3, 1989; Tillett and Neilan, Biotechniques, 24:568-70, 572, 1998;http://www.qiagen.com/literature/handbooks/plkmini/plm_(—)399.pdf) ofthe selected LTVEC and re-transform the miniprep LTVEC DNA into E. coliusing electroporation (Sambrook, J., E. F. Fritsch and T. Maniatis,Molecular Cloning: A Laboratory Manual, Second Edition, Vols 1, 2, and3, 1989). This step is necessary to get rid of the plasmid encoding therecombinogenic proteins that are utilized for the bacterial homologousrecombination step (Zhang et al., Nat Genet, 20:123-8, 1998; Narayananet al., Gene Ther, 6:442-7, 1999). It is useful to get rid of thisplasmid (a) because it is a high copy number plasmid and may reduce theyields obtained in the large scale LTVEC preps; (b) to eliminate thepossibility of inducing expression of the recombinogenic proteins; and(c) because it may obscure physical mapping of the LTVEC. Beforeintroducing the LTVEC into eukaryotic cells, larger amounts of LTVEC DNAare prepared by standard methodology(http://www.qiagen.com/literature/handbooks/plk/plklow.pdf; Sambrook,J., E. F. Fritsch And T. Maniatis. Molecular Cloning: A LaboratoryManual, Second Edition, Vols 1, 2, and 3, 1989; Tillett and Neilan,Biotechniques, 24:568-70, 572, 1998). However, this step can be bypassedif a bacterial homologous recombination method that utilizes arecombinogenic prophage is used, i.e. where the genes encoding therecombinogenic proteins are integrated into the bacterial chromosome(Yu, et al., Proc Natl Acad Sci USA, 97:5978-83, 2000), is used.

b. Linearizing the LTVEC DNA:

To prepare the LTVEC for introduction into eukaryotic cells, the LTVECis preferably linearized in a manner that leaves the modified endogenousgene(s) or chromosomal locus (loci) DNA flanked with long homology arms.This can be accomplished by linearizing the LTVEC, preferably in thevector backbone, with any suitable restriction enzyme that digests onlyrarely. Examples of suitable restriction enzymes include NotI, PacI,SfiI, SrfI, SwaI, FseI, etc. The choice of restriction enzyme may bedetermined experimentally (i.e. by testing several different candidaterare cutters) or, if the sequence of the LTVEC is known, by analyzingthe sequence and choosing a suitable restriction enzyme based on theanalysis. In situations where the LTVEC has a vector backbone containingrare sites such as CosN sites, then it can be cleaved with enzymesrecognizing such sites, for example λ terminase (Shizuya et al., ProcNatl Acad Sci USA, 89:8794-7, 1992; Becker and Gold, Proc Natl Acad SciUSA, 75:4199-203, 1978; Rackwitz et al., Gene, 40:259-66, 1985).

Step 5. Introduction of LTVEC into Eukaryotic Cells and Selection ofCells where Successful Introduction of the LTVEC has Taken Place.

LTVEC DNA can be introduced into eukaryotic cells using standardmethodology, such as transfection mediated by calcium phosphate, lipids,or electroporation (Sambrook, J., E. F. Fritsch And T. Maniatis.Molecular Cloning: A Laboratory Manual, Second Edition, Vols 1, 2, and3, 1989). The cells where the LTVEC has been introduced successfully canbe selected by exposure to selection agents, depending on the selectablemarker gene that has been engineered into the LTVEC. As a non-limitingexample, if the selectable marker is the neomycin phosphotransferase(neo) gene (Beck, et al., Gene, 19:327-36, 1982), then cells that havetaken up the LTVEC can be selected in G418-containing media; cells thatdo not have the LTVEC will die whereas cells that have taken up theLTVEC will survive (Santerre, et al., Gene, 30:147-56, 1984). Othersuitable selectable markers include any drug that has activity ineukaryotic cells (Joyner, The Practical Approach Series, 293, 1999),such as hygromycin B (Santerre, et al., Gene, 30:147-56, 1984; Bernard,et al., Exp Cell Res, 158:237-43, 1985; Giordano and McAllister, Gene,88:285-8, 1990), Blasticidin S (Izumi, et al., Exp Cell Res, 197:229-33,1991), and other which are familiar to those skilled in the art.

Step 6. Screen for Homologous Recombination Events in Eukaryotic CellsUsing Quantitative Assay for Modification of Allele (MOA).

Eukaryotic cells that have been successfully modified by targeting theLTVEC into the locus of interest can be identified using a variety ofapproaches that can detect modification of allele within the locus ofinterest and that do not depend on assays spanning the entire homologyarm or arms. Such approaches can include but are not limited to:

(a) quantitative PCR using TaqMan® (Lie and Petropoulos, Curr OpinBiotechnol, 9:43-8, 1998);(b) quantitative MOA assay using molecular beacons (Tan, et al.,Chemistry, 6:1107-11, 2000)(c) fluorescence in situ hybridization FISH (Laan, et al., Hum Genet,96:275-80, 1995) or comparative genomic hybridization (CGH) (Forozan, etal., Trends Genet, 13:405-9, 1997; Thompson and Gray, J Cell BiochemSuppl, 139-43, 1993; Houldsworth and Chaganti, Am Pathol, 145:1253-60,1994);(d) isothermic DNA amplification (Lizardi, et al., Nat Genet, 19:225-32,1998; Mitra and Church, Nucleic Acids Res, 27:e34, 1999);(e) quantitative hybridization to an immobilized probe(s) (Southern, J.Mol. Biol. 98: 503, 1975; Kafatos F C; Jones C W; Efstratiadis A,Nucleic Acids Res 7(6):1541-52, 1979);

(f) Invader Probes® (Third Wave Technologies);

(g) Eclipse™ and Molecular Beacon probes (Synthetic Genetics); and(h) MMP assays (High Throughput Genomics)

Applicants provide herein an example in which TaqMan® quantitative PCRis used to screen for successfully targeted eukaryotic cells. In thisnon-limiting example, TaqMan® is used to identify eukaryotic cells,which have undergone homologous recombination wherein a portion of oneof two endogenous alleles in a diploid genome has been replaced byanother sequence. In contrast to traditional methods, in which adifference in restriction fragment length spanning the entire homologyarm or arms indicates the modification of one of two alleles, thequantitative TaqMan® method will detect the modification of one alleleby measuring the reduction in copy number (by half) of the unmodifiedallele. Specifically, the probe detects the unmodified allele and notthe modified allele. Therefore, the method is independent of the exactnature of the modification and not limited to the sequence replacementdescribed in this example. TaqMan is used to quantify the number ofcopies of a DNA template in a genomic DNA sample, especially bycomparison to a reference gene (Lie and Petropoulos, Curr OpinBiotechnol, 9:43-8, 1998). The reference gene is quantitated in the samegenomic DNA as the target gene(s) or locus (loci). Therefore, twoTaqMan® amplifications (each with its respective probe) are performed.One TaqMan® probe determines the “Ct” (Threshold Cycle) of the referencegene, while the other probe determines the Ct of the region of thetargeted gene(s) or locus (loci) which is replaced by successfultargeting. The Ct is a quantity that reflects the amount of starting DNAfor each of the TaqMan® probes, i.e. a less abundant sequence requiresmore cycles of PCR to reach the threshold cycle. Decreasing by half thenumber of copies of the template sequence for a TaqMan® reaction willresult in an increase of about one Ct unit. TaqMan® reactions in cellswhere one allele of the target gene(s) or locus (loci) has been replacedby homologous recombination will result in an increase of one Ct for thetarget TaqMan® reaction without an increase in the Ct for the referencegene when compared to DNA from non-targeted cells. This allows for readydetection of the modification of one allele of the gene(s) of interestin eukaryotic cells using LTVECs.

As stated above, modification of allele (MOA) screening is the use ofany method that detects the modification of one allele to identifycells, which have undergone homologous recombination. It is not arequirement that the targeted alleles be identical (homologous) to eachother, and in fact, they may contain polymorphisms, as is the case inprogeny resulting from crossing two different strains of mice. Inaddition, one special situation that is also covered by MOA screening istargeting of genes which are normally present as a single copy in cells,such as some of the located on the sex chromosomes and in particular, onthe Y chromosome. In this case, methods that will detect themodification of the single targeted allele, such as quantitative PCR,Southern blottings, etc., can be used to detect the targeting event. Itis clear that the method of the invention can be used to generatemodified eukaryotic cells even when alleles are polymorphic or when theyare present in a single copy in the targeted cells.

Step 8. Uses of Genetically Modified Eukaryotic Cells.

(a) The genetically modified eukaryotic cells generated by the methodsdescribed in steps 1 through 7 can be employed in any in vitro or invivo assay, where changing the phenotype of the cell is desirable.(b) The genetically modified eukaryotic cell generated by the methodsdescribed in steps 1 through 7 can also be used to generate an organismcarrying the genetic modification. The genetically modified organismscan be generated by several different techniques including but notlimited to:

-   -   1. Modified embryonic stem (ES) cells such as the frequently        used rat and mouse ES cells. ES cells can be used to create        genetically modified rats or mice by standard blastocyst        injection technology or aggregation techniques (Robertson,        Practical Approach Series, 254, 1987; Wood, et al., Nature,        365:87-9, 1993; Joyner, The Practical Approach Series, 293,        1999), tetraploid blastocyst injection (Wang, et al., Mech Dev,        62:137-45, 1997), or nuclear transfer and cloning (Wakayama, et        al., Proc Natl Acad Sci USA, 96:14984-9, 1999). ES cells derived        from other organisms such as rabbits (Wang, et al., Mech Dev,        62:137-45, 1997; Schoonjans, et al., Mol Reprod Dev,        45:439-43, 1996) or chickens (Pain, et al., Development,        122:2339-48, 1996) or other species should also be amenable to        genetic modification(s) using the methods of the invention.    -   2. Modified protoplasts can be used to generate genetically        modified plants (for example see U.S. Pat. No. 5,350,689 “Zea        mays plants and transgenic Zea mays plants regenerated from        protoplasts or protoplast-derived cells”, and U.S. Pat. No.        5,508,189 “Regeneration of plants from cultured guard cell        protoplasts” and references therein).    -   3. Nuclear transfer from modified eukaryotic cells to oocytes to        generate cloned organisms with modified allele (Wakayama, et        al., Proc Natl Acad Sci USA, 96:14984-9, 1999; Baguisi, et al.,        Nat Biotechnol, 17:456-61, 1999; Wilmut, et al., Reprod Fertil        Dev, 10:639-43, 1998; Wilmut, et al., Nature, 385:810-3, 1997;        Wakayama, et al., Nat Genet, 24:108-9, 2000; Wakayama, et al.,        Nature, 394:369-74, 1998; Rideout, et al., Nat Genet, 24:109-10,        2000; Campbell, et al., Nature, 380:64-6, 1996).    -   4. Cell-fusion to transfer the modified allele to another cell,        including transfer of engineered chromosome(s), and uses of such        cell(s) to generate organisms carrying the modified allele or        engineered chromosome(s) (Kuroiwa, et al., Nat Biotechnol,        18:1086-1090, 2000).    -   5. The method of the invention are also amenable to any other        approaches that have been used or yet to be discovered.

While many of the techniques used in practicing the individual steps ofthe methods of the invention are familiar to the skilled artisan,Applicants contend that the novelty of the method of the invention liesin the unique combination of those steps and techniques coupled with thenever-before-described method of introducing a LTVEC directly intoeukaryotic cells to modify a chromosomal locus, and the use ofquantitative MOA assays to identify eukaryotic cells which have beenappropriately modified. This novel combination represents a significantimprovement over previous technologies for creating organisms possessingmodifications of endogenous genes or chromosomal loci.

EXAMPLES Example 1 Engineering Mouse ES Cells Bearing a Deletion of theOCR10 Gene

a. Selection of a Large Genomic DNA Clone Containing mOCR10.

A Bacterial Artificial Chromosome (BAC) clone carrying a large genomicDNA fragment that contained the coding sequence of the mouse OCR10(mOCR10) gene was obtained by screening an arrayed mouse genomic DNA BAClibrary (Incyte Genomics) using PCR. The primers employed to screen thislibrary were derived from the mOCR10 gene cDNA sequence.

Two primer pairs where used:

(a) OCR10.RAA (5′-AGCTACCAGCTGCAGATGCGGGCAG-3′; SEQ ID NO: 1) andOCR10.PVIrc (5′-CTCCCCAGCCTGGGTCTGAAAGATGACG-3′; SEQ ID NO: 2)which amplifies a 102 bp DNA; and (b) OCR10.TDY(5′-GACCTCACTTGCTACACTGACTAC-3′; SEQ ID NO: 3) and OCR10.QETrc(5′-ACTTGTGTAGGCTGCAGAAGGTCTCTTG-3′; SEQ ID NO: 4)which amplifies a 1500 bp DNA.

This mOCR10 BAC contained approximately 180 kb of genomic DNA includingthe complete mOCR10 coding sequence. This BAC clone was used to generatean LTVEC which was subsequently used to delete a portion of the codingregion of mOCR10 while simultaneously introducing a reporter gene whoseinitiation codon precisely replaced the initiation codon of OCR10, aswell as insertion of a selectable marker gene useful for selection bothin E. coli and mammalian cells following the reporter gene (FIG. 2). Thereporter gene (in this non-limiting example LacZ, the sequence of whichis readily available to the skilled artisan), encodes the E. coliβ-galactosidase enzyme. Because of the position of insertion of LacZ(its initiating codon is at the same position as the initiation codon ofmOCR10) the expression of lacZ should mimic that of mOCR10, as has beenobserved in other examples where similar replacements with LacZ wereperformed using previous technologies (see “Gene trap strategies in EScells”, by W Wurst and A. Gossler, in Joyner, The Practical ApproachSeries, 293, 1999). The LacZ gene allows for a simple and standardenzymatic assay to be performed that can reveal its expression patternsin situ, thus providing a surrogate assay that reflects the normalexpression patterns of the replaced gene(s) or chromosomal locus (loci).

b. Construction of Donor Fragment and Generation of LTVEC.

The modification cassette used in the construction of the mOCR10 LTVECis the lacZ-SV40 polyA-PGKp-EM7-neo-PGK polyA cassette wherein lacZ is amarker gene as described above, SV40 polyA is a fragment derived fromSimian Virus 40 (Subramanian, et al., Prog Nucleic Acid Res Mol Biol,19:157-64, 1976; Thimmappaya, et al., J Biol Chem, 253:1613-8, 1978;Dhar, et al., Proc Nall Acad Sci USA, 71:371-5, 1974; Reddy, et al.,Science, 200:494-502, 1978) and containing a polyadenylation site andsignal (Subramanian, et al., Prog Nucleic Acid Res Mol Biol, 19:157-64,1976; Thimmappaya, et al., J Biol Chem, 253:1613-8, 1978; Dhar, et al.,Proc Nall Acad Sci USA, 71:371-5, 1974; Reddy, et al., Science,200:494-502, 1978), PGKp is the mouse phosphoglycerate kinase (PGK)promoter (Adra, et al., Gene, 60:65-74, 1987) (which has been usedextensively to drive expression of drug resistance genes in mammaliancells), EM7 is a strong bacterial promoter that has the advantage ofallowing for positive selection in bacteria of the completed LTVECconstruct by driving expression of the neomycin phosphotransferase (neo)gene, neo is a selectable marker that confers Kanamycin resistance inprokaryotic cells and G418 resistance in eukaryotic cells (Beck, et al.,Gene, 19:327-36, 1982), and PGK polyA is a 3′ untranslated regionderived from the PGK gene and containing a polyadenylation site andsignal (Boer, et al., Biochem Genet, 28:299-308, 1990).

To construct the mOCR10 LTVEC, first a donor fragment was generatedconsisting of a mOCR10 homology box 1 (hb1) attached upstream from theLacZ gene in the modification cassette and a mOCR10 homology box 2 (hb2)attached downstream of the neo-PGK polyA sequence in the modificationcassette (FIG. 2), using standard recombinant genetic engineeringtechnology. Homology box 1 (hb1) consists of 211 bp of untranslatedsequence immediately upstream of the initiating methionine of the mOCR10open reading frame (mOCR10 ORF) (FIG. 3A-3D). Homology box 2 (hb2)consists of last 216 bp of the mOCR10 ORF, ending at the stop codon(FIG. 3A-3D).

Subsequently, using bacterial homologous recombination (Zhang, et al.,Nat Genet, 20:123-8, 1998; Angrand, et al., Nucleic Acids Res, 27:e16,1999; Muyrers, et al., Nucleic Acids Res, 27:1555-7, 1999; Narayanan, etal., Gene Ther, 6:442-7, 1999; Yu, et al., Proc Nall Acad Sci USA,97:5978-83, 2000), this donor fragment was used to precisely replace themOCR10 coding region (from initiation methionine to stop codon) with theinsertion cassette, resulting in construction of the mOCR10 LTVEC (FIG.2). Thus, in this mOCR10 LTVEC, the mOCR10 coding sequence was replacedby the insertion cassette creating an approximately 20 kb deletion inthe mOCR10 locus while leaving approximately 130 kb of upstream homology(upstream homology arm) and 32 kb of downstream homology (downstreamhomology arm).

It is important to note that LTVECs can be more rapidly and convenientlygenerated from available BAC libraries than targeting vectors made usingprevious technologies because only a single bacterial homologousrecombination step is required and the only sequence informationrequired is that needed to generate the homology boxes. In contrast,previous approaches for generating targeting vectors using bacterialhomologous recombination require that large targeting vectors be“trimmed” prior to their introduction in ES cells (Hill et al.,Genomics, 64:111-3, 2000). This trimming is necessary because of theneed to generate homology arms short enough to accommodate the screeningmethods utilized by previous approaches. One major disadvantage of themethod of Hill et al. is that two additional homologous recombinationsteps are required simply for trimming (one to trim the region upstreamof the modified locus and one to trim the region downstream of themodified locus). To do this, substantially more sequence information isneeded, including sequence information spanning the sites of trimming.

In addition, another obvious advantage, illustrated by the aboveexample, is that a very large deletion spanning the mOCR10 gene(approximately 20 kb) can be easily generated in a single step. Incontrast, using previous technologies, to accomplish the same task mayrequire several steps and may involve marking the regions upstream anddownstream of the coding sequences with loxP sites in order to use theCre recombinase to remove the sequence flanked by these sites afterintroduction of the modified locus in eukaryotic cells. This may beunattainable in one step, and thus may require the construction of twotargeting vectors using different selection markers and two sequentialtargeting events in ES cells, one to introduce the loxP site at theregion upstream of the coding sequence and another to introduce the loxPsite at the region downstream of the coding sequence. It should befurther noted that the creation of large deletions often occurs with lowefficiency using the previous targeting technologies in eukaryoticcells, because the frequency of achieving homologous recombination maybe low when using targeting vectors containing large deletion flanked byrelatively short homology arms. The high efficiency obtained using themethod of the invention (see below) is due to the very long homologyarms present in the LTVEC that increase the rate of homologousrecombination in eukaryotic cells.

c. Verification, Preparation, and Introduction of mOCR10 LTVEC DNA intoES Cells.

The sequence surrounding the junction of the insertion cassette and thehomology sequence was verified by DNA sequencing. The size of the mOCR10LTVEC was verified by restriction analysis followed by pulsed field gelelectrophoresis (PFGE) (Cantor, et al., Annu Rev Biophys Biophys Chem,17:287-304, 1988; Schwartz and Cantor, Cell, 37:67-75, 1984). A standardlarge-scale plasmid preparation of the mOCR10 LTVEC was done, theplasmid DNA was digested with the restriction enzyme NotI, which cuts inthe vector backbone of the mOCR10 LTVEC, to generate linear DNA.Subsequently the linearized DNA was introduced into mouse ES cells byelectroporation (Robertson, Practical Approach Series, 254, 1987;Joyner, The Practical Approach Series, 293, 1999; Sambrook, et al.,Sambrook, J., E. F. Fritsch and T. Maniatis. Molecular Cloning: ALaboratory Manual, Second Edition, Vols 1, 2, and 3, 1989). ES cellssuccessfully transfected with the mOCR10 LTVEC were selected for inG418-containing media using standard selection methods (Robertson,Practical Approach Series, 254, 1987; Joyner, The Practical ApproachSeries, 293, 1999).

d. Identification of Targeted ES Cells Clones Using a QuantitativeModification of Allele (MOA) Assay.

To identify ES cells in which one of the two endogenous mOCR10 genes hadbeen replaced by the modification cassette sequence, DNA from individualES cell clones was analyzed by quantitative PCR using standard TaqMan®methodology as described (Applied Biosystems, TaqMan® Universal PCRMaster Mix, catalog number P/N 4304437; see alsohttp://www.pebiodocs.com/pebiodocs/04304449.pdf). The primers andTaqMan® probes used are as described in FIG. 3A-3D. A total of 69independent ES cells clones where screened and 3 were identified aspositive, i.e. as clones in which one of the endogenous mOCR10 codingsequence had been replaced by the modification cassette described above.

Several advantages of the MOA approach are apparent:

(i) It does not require the use of a probe outside the locus beingmodified, thus obviating the need to know the sequence flanking themodified locus.(ii) It requires very little time to perform compared to conventionalSouthern blot methodology which has been the previous method of choice(Robertson, Practical Approach Series, 254, 1987, Joyner, The PracticalApproach Series, 293, 1999), thus reducing the time for identifyingcorrectly modified cells from the typical several days to just a fewhours.

This is a significant improvement in the way screening has beenperformed in the past and makes it a much less labor-intensive and morecost-effective approach to screening for homologous recombination eventsin eukaryotic cells.

Yet another advantage of the method of the invention is that it is alsosuperior to previous technologies because of its ability to targetdifficult loci. Using previous technologies, it has been shown that forcertain loci the frequency of successful targeting may by as low as 1 in2000 integration events, perhaps even lower. Using the method of theinvention, Applicants have demonstrated that such difficult loci can betargeted much more efficiently using LTVECs that contain long homologyarms (i.e. greater than those allowed by previous technologies). As thenon-limiting example described above demonstrates, the Applicants havetargeted the OCR10 locus, a locus that has previously provenrecalcitrant to targeting using conventional technology. Using themethod of the invention, Applicants have shown that they have obtainedsuccessful targeting in 3 out of 69 ES cells clones in which the mOCR10LTVEC (containing more than 160 kb of homology arms, and introducing a20 kb deletion) had integrated, whereas using previous technology for EScell targeting (Joyner, The Practical Approach Series, 293, 1999) usinga plasmid-based vector with homology arms shorter than 10-20 kb whilealso introducing a deletion of less than 15 kb, no targeted events wereidentified among more than 600 integrants of the vector. These dataclearly demonstrate the superiority of the method of the invention overprevious technologies.

Example 2 Increased Targeting Frequency and Abrogation of the Need toUse Isogenic DNA when LTVECs are Used as the Targeting Vectors

As noted above, the increased targeting frequency obtained using longhomology aims should diminish the benefit, if any, derived from usinggenomic DNA in constructing LTVECs that is isogenic with (i.e. identicalin sequence to) the DNA of the eukaryotic cell being targeted. To testthis hypothesis, Applicants have constructed several LTVECs usinggenomic DNA derived from the same mouse substrain as the eukaryotic cellto be targeted (presumably isogenic), and a large number of other LTVECsusing genomic DNA derived from mouse substrains differing from that ofthe eukaryotic cell to be targeted (presumably non-isogenic). Thenon-isogenic LTVECs exhibited an average targeting frequency of 6%(ranging from 1-20%, Table 1), while the isogenic LTVECs exhibited asaverage targeting frequency of 3% (ranging from 2-5%), indicating thatthe rate of successful targeting using LTVECs does not depend onisogenicity.

TABLE 1 Approximate Size (kb) Target Gene Description DNA Origin ES CellBAC Size Arm 1 Arm 2 Deletion Positive Clones % Targeting NON-ISOGENICOGH LacZ-ATG fusion SvJ CJ7 148 50 90 5 4 4 OCR10(A) LacZ-ATG fusion SvJCJ7 165 135 8 20 1 1.4 OCR10(B) LacZ-ATG fusion SvJ CJ7 160 130 32 20 34.3 MA61 LacZ-ATG fusion SvJ CJ7 95 N/D N/D 30 3 4.6 MA16 LacZ-ATGfusion SvJ CJ7 120 N/D N/D 8 8 13 AGRP LacZ-ATG fusion SvJ CJ7 189 14732 8 1 1.1 SHIP-2 LacZ-ATG fusion SvJ CJ7 136 30 90 11 7 15 Sm22LacZ-ATG fusion SvJ CJ7 70 35 35 0.9 18 20 LGR7L LacZ-ATG fusion SvJ CJ7200 N/D N/D 1 3 3.2 C5aR LacZ-ATG fusion SvJ CJ7 160 80 25 1 4 4.2 IL18LacZ-ATG fusion SvJ CJ7 120 50 65 10 7 7.3 PLGF LacZ-ATG fusion SvJ CJ7130 40 20 8 1 1 NaDC-1 LacZ-ATG fusion SvJ CJ7 180 30 45 25 4 2.1ISOGENIC ROR1 Intracell-LacZ fusion CJ7 CJ7 55 14 14 20 5 5 ROR1Intracell-3xmyc fusion CJ7 CJ7 55 14 14 20 2 2 ROR2 Brachydactylymutation CJ7 CJ7 45 11 24 0.5 2 2 and Myc tag

Example 3 Detailed Description of the TaqMan®-Based MOA forIdentification of Targeted ES Clones

ES cell clones that have taken up the LTVEC and incorporated it into thegenome at the targeted locus by homologous recombination are identifiedby a modification of allele (MOA) assay that uses real-time quantitativePCR to discern the difference between targeted ES cell clones, in whichone of the two targeted alleles is modified, and non-targeted ES cellclones, in which both alleles remain unmodified. The MOA assay consistsof a primary and a secondary screen. The primary screen contains thefollowing steps: (1) growth of LTVEC-transfected ES cell clones ongelatin-coated 96-well plates; (2) isolation of genomic DNA from each EScell clone; (3) use of each genomic DNA sample as a template in 8separate quantitative PCRs on two 384-well plates in which 2 of the PCRsemploy a target-locus-specific primer set that hybridyzes to DNAsequences at one end of the genomic fragment targeted for deletion(‘upstream PCR’), 2 of the PCRs employ a target-locus-specific primerset that hybridyzes to DNA sequences at the other end of the genomicfragment targeted for deletion (‘downstream PCR’), 4 of the PCRs employprimer sets that recognize four non-targeted reference loci (‘referencePCRs’), and each PCR includes a fluorescent probe (for example a TaqMan®[ABI], Eclipse™, or Molecular Beacon probe [Synthetic Genetics]) thatrecognizes the amplified sequence and whose fluorescence signal isdirectly proportional to the amount of PCR product; (4) running the PCRsin a device that combines a thermocycler with a fluorescence detector(for example the ABI 7900HT) that quantifies the accumulation ofamplification products during the PCR and determines the threshold cycle(C_(T)), the point in the PCR at which the fluorescence signal isdetectable above background noise; (5) for each ES cell clone DNAsample, calculation of the difference in the C_(T) values (ΔC_(T))between the upstream PCRs and each of the four reference PCRs andbetween the downstream PCRs and each of the four reference PCRs tocreate 8 tables of 96 ΔC_(T) values; (6) normalization of the ΔC_(T)values to positive values; (7) calculation of the median ΔC_(T) valuefor each target-reference comparison table; (8) determination of aconfidence score by use of a computer program that examines the eightΔC_(T) tables and calculates the number of times a given ES cell cloneDNA sample produces a ACT value within the tolerance ranges 0.5 to 1.5,0.25 to 1.5, 0.5 to 2.0, 0.25 to 2.0, 0.5 to 3.0 and 0.25 to 3.0 cyclesgreater than the median ΔC_(T) (examples of computer programminglanguages suitable for creating or writing such a program include visualbasics, Java, or any other computer programming language familiar to theskilled artisan); (9) plotting the values and their medians for each ofthe eight ACT tables as histograms; and (10) identification of correctlytargeted ES cell clone candidates from an inspection of the confidencescores and the ΔC_(T) histograms. In a preferred example, the ΔC_(T)value for the candidate targeted clone falls within 0.5 to 1.5 cyclesgreater than the median in 8 out of 8 reference comparisons.

Candidate clones identified by the MOA assay primary screen areconfirmed or rejected in a secondary screen, which contains thefollowing steps: (1) use of the genomic DNA from each of the positivecandidate ES cell clones, from a larger number of negative clones, andfrom genomic DNA copy-number standards from mice that carry one or twocopies of the LTVEC LacZ-Neo cassette per diploid genome as templates in8 separate quantitative PCRs on two 384-well plates in which 1 reactionis an upstream PCR (as in the primary screen), one reaction is adownstream PCR (as in the primary screen), 4 reactions are referencePCRs with two reference loci that are different from those used in theprimary screen, one reaction is a PCR with primers and a probe that arespecific for the LacZ gene of the LTVEC, and one reaction is a PCR withprimers and a probe that are specific for the Neo gene of the LTVEC; (2)running the PCRs in a quantitative PCR device, as in the primary screen;(3) calculation, as in the primary screen, of the ΔC_(T) values betweenthe upstream PCR and each of the two reference PCRs, between thedownstream PCRs and each of the two reference PCRs, between the LacZ PCRand each of the two reference PCRs, and between the Neo PCR and each ofthe two reference PCRs to create eight ΔC_(T) tables; (4) normalizationof the ΔC_(T) values to positive values; (5) calculation of the medianvalue for each ΔC_(T) table; (6) calculation of confidence scores as inthe primary screen; and (7) plotting the values and their medians foreach of the eight ΔC_(T) tables as histograms.

From an inspection of the confidence scores and the ΔC_(T) histogramsfor both the primary and secondary screens, correctly targeted ES clonecandidates are either confirmed or rejected. In a preferred example, theΔC_(T) value for the candidate targeted clone falls within 0.5 to 1.5cycles greater than the median in 12 out of 12 reference comparisonsfrom the combined primary and secondary screens.

To score the number of copies of the LTVEC per diploid genome in theconfirmed, correctly targeted ES clones, their ΔC_(T) values from thecomparisons of the LacZ and Neo PCRs with the two reference PCRs arecompared with the ΔC_(T) values for the LacZ-Neo copy number standards.Each ES cell clone is scored as having 1, 2 or greater than 2 copies ofthe LTVEC. For each modified allele project, ES cell clones are screenedin groups of 96 (usually fewer than 288 total clones) until 3 clonesthat score positive in the MOA assay and have a single copy of theLacZ-Neo cassette are identified.

Example 4 Use of FISH to Identify Correctly Targeted LTVECs in ES Cells

Using the LTVEC technology described herein, Applicants knocked out theSM22alpha gene in ES cells. SM22alpha is a 22-kDa smooth muscle cell(SMC) lineage-restricted protein that physically associates withcytoskeletal actin filament bundles in contractile SMCs. The targeted EScells were then subjected to standard fluorescence in situ hybridization(FISH) on metaphase chromosomal spreads to verify that the gene wasappropriately targeted. The experiment was performed with two probes: 1)an SM22alpha gene probe consisting of the unmodified SM22alpha BAC cloneused to generate the LTVEC and 2) a LacZ and Neomycin DNA probe whichdetects only the gene modification made by the targeting event(insertion of LacZ and Neo gene cassettes). Metaphase chromosomalspreads were prepared from cells and hybridization was performedsimultaneously with both probes, which were labeled with differentcolored fluorophores to allow detection of hybridization of each probewithin the same spread. A non-targeted ES cell line was analyzed inparallel as a control. As expected, in the control spreads, two allelesof SM22alpha were detected on homologous chromosomal arms, but there wasno hybridization of the LacZ-Neo probe. As in controls, in targeted EScell spreads two alleles were also detected at the same chromosomallocation and on homologous chromosomes, but double-labeling with theLacZ-Neo probe was apparent on one of the two chromosomes indicatingco-localization of the SM22alpha and LacZ-Neo DNA sequences at thatallele of SM22alpha. Importantly, no SM22alpha or LacZ-Neo genesequences were detected at inappropriate locations in the spreads. Lackof extra integration of SM22alpha gene sequences and co-localization ofLacZ-Neo with SM22alpha in one chromosome of a homologous pair stronglysuggests that correct targeting of LacZ-Neo to one of the SM22alphaalleles via homologous recombination had occurred.

Example 5 Lowering the Amount of DNA Used to Electroporate ES CellsImproves Targeting Efficiency

Standard methods for targeted modification of genes in mouse embryonicstem (ES) cells typically employ 20 to 40 μg of targeting vector in theelectroporation procedure. Applicants have discovered that with LTVECs,electroporation with much lower amounts of DNA—in the range of about 1to 5 μg per 1×10⁷ cells—doubles the frequency of correctly targetedhomologous recombination events while greatly reducing the number ofsecondary, non-homologous insertion events. This clear improvement intargeting efficiency is important because it significantly reduces thenumber of ES cells clones that need to be screened to find severalpositive clones with a correctly targeted, single-copy modification. Theassociated benefits are reduced cost and increased throughput.

Example 6 Use of the Method of the Invention to Create MA61 KnockoutMice to Study Muscle Atrophy

MA61, also called MAFbx, is a recently discovered ubiquitin ligase thatis up-regulated in various conditions of muscle atrophy (See U.S.Provisional Application No. 60/264,926, filed Jan. 30, 2001, U.S.Provisional Application No. 60/311,697, filed Aug. 10, 2001, and U.S.Provisional Application (serial number not yet known), filed Oct. 22,2001, all assigned to Regeneron Pharmaceuticals, Inc., each of which isincorporated herein in its entirety by reference). To further study thebiological significance of this gene in muscle atrophy, knockout micewere created using the method of the invention as follows.

First, to obtain a large cloned genomic fragment containing the MA61gene, a Bacterial Artificial Chromosome (BAC) library was screened withprimers derived from the MA61 cDNA sequence. The BAC clone thus obtainedwas then used to create a Large Targeting Vector for Eukaryotic Cells(LTVEC) as follows. A modification cassette containing a 5′ homologybox/lacZ gene/polyA/PGK promoter/neo/polyA/3′ homology box wasengineered. The homology boxes were appended to mark the sites ofbacterial homologous recombination during the generation of the LTVEC.The LacZ is a reporter gene that was positioned such that its initiatingcodon was at the same position as the initiating codon of MA61.Following homologous recombination in bacteria, the modificationcassette replaced the MA61 gene. Thus, a MA61 LTVEC was created whereinthe MA61 coding sequences in the BAC clone was replaced by themodification cassette engineered as described supra. LTVEC DNA was thenprepared, purified, and linearized for introduction into eukaryoticcells as described infra.

A MA61 LTVEC DNA miniprep was prepared (Sambrook, J., E. F. Fritsch AndT. Maniatis. Molecular Cloning: A Laboratory Manual, Second Edition,Vols 1, 2, and 3, 1989; Tillett and Neilan, Biotechniques, 24:568-70,572, 1998;http://www.qiagen.com/literature/handbooks/plkmini/plm_(—)399.pdf) andre-transformed into E. coli using electroporation (Sambrook, J., E. F.Fritsch and T. Maniatis, Molecular Cloning: A Laboratory Manual, SecondEdition, Vols 1, 2, and 3, 1989) in order to get rid of the plasmidencoding the recombinogenic proteins that are utilized for the bacterialhomologous recombination step (Zhang et al., Nat Genet, 20:123-8, 1998;Narayanan et al., Gene Ther, 6:442-7, 1999). Before introducing the MA61LTVEC into eukaryotic cells, larger amounts of MA61 LTVEC were preparedby standard methodology(http://www.qiagen.com/literature/handbooks/plk/plklow.pdf; Sambrook,J., E. F. Fritsch And T. Maniatis. Molecular Cloning: A LaboratoryManual, Second Edition, Vols 1, 2, and 3, 1989; Tillett and Neilan,Biotechniques, 24:568-70, 572, 1998).

Next, to prepare the MA61 LTVEC for introduction into eukaryotic cells,the MA61 LTVEC was linearized. This was accomplished by digesting withthe restriction enzyme NotI, which leaves the modified endogenousgene(s) or chromosomal locus (loci) DNA flanked with long homology aims.

The MA61 LTVEC was then introduced into eukaryotic cells using standardelectroporation methodology (Sambrook, J., E. F. Fritsch And T.Maniatis. Molecular Cloning: A Laboratory Manual, Second Edition, Vols1, 2, and 3, 1989)). The cells in which the MA61 LTVEC was introducedsuccessfully were selected by exposure to a selection agent. Because theselectable marker used in the modification cassette was the neomycinphosphotransferase (neo) gene (Beck, et al., Gene, 19:327-36, 1982), thecells that had taken up the MA61 LTVEC were selected in a mediumcontaining G418; cells that do not have the MA61 LTVEC died whereascells that have taken up the MA61 LTVEC survived (Santerre, et al.,Gene, 30:147-56, 1984).

Eukaryotic cells that have been successfully modified by targeting theMA61 LTVEC into the MA61 locus were identified with the quantitative PCRmethod TaqMan® (Lie and Petropoulos, Curr Opin Biotechnol, 9:43-8,1998).

Finally, the genetically modified ES cells were used to creategenetically modified, in this case knock out, mice by standardblastocyst injection technology. Thus created were the MA61 knock-outs,mice in which the MA61 gene had been deleted.

Both of these knock out mice and wild-type (WT) mice were exposed toatrophy-inducing conditions, created by denervating the mice, and levelsof atrophy compared. First, the sciatic nerve was isolated in themid-thigh region of the right hind limb and transected in the mice.Transection of the sciatic nerve leads to denervation and, over afourteen-day period, to atrophy in the muscles of the lower limb,specifically the tibialis anterior and gastrocnemius muscles, over a14-day period. At 7 and 14 days following the denervation, animals weresacrificed by carbon dioxide inhalation. Then the tibialis anterior (TA)and gastrocnemius complex (GA) were removed from the right (denervated)and left (intact) hind limbs, weighed, and frozen at a fixed length inliquid nitrogen cooled isopentane. The amount of atrophy was assessed bycomparing the weight of the muscles from the denervated limb with theweight of the muscles from the non-denervated limb.

Muscle atrophy was assessed 7 and 14 days following transection of theright sciatic nerve. The wet weights of the right, denervated muscleswere compared to the wet weights of the left, non-denervated muscles.The right:left comparisons are given in Table 2.

Gastrocnemius Complex Tibialis Anterior Genotype Sample size Mean SESample size Mean SE 7 days WT 7 0.76 0.016 11 0.68 0.033 KO 6 0.84 0.02211 0.80 0.015 14 days WT 5 0.55 0.024 5 0.62 0.023 KO 5 0.80 0.019 50.80 0.012

At 7 and 14 days, the muscles from the knock mice showed significantly(p<0.001) less atrophy than the muscles from the wild type mice. Thedifference between the knock out and wild type mice was greater at 14days than at 7 days. While the wild type mice continued to atrophybetween 7 and 14 days, the knock out mice showed no additional atrophy.

In summary, the approach of creating LTVECs and directly using them astargeting vectors combined with MOA screening for homologousrecombination events in ES cells creates a novel method for engineeringgenetically modified loci that is rapid, inexpensive and represents asignificant improvement over the tedious, time-consuming methodspreviously in use. It thus opens the possibility of a rapid large scalein vivo functional genomics analysis of essentially any and all genes inan organism's genome in a fraction of the time and cost necessitated byprevious methodologies.

Although the foregoing invention has been described in some detail byway of illustration and examples, it will be readily apparent to thoseof ordinary skill in the art that certain changes and modifications maybe made to the teachings of the invention without departing from thespirit or scope of the appended claims.

We claim:
 1. A method for targeting a large cloned genomic fragment to adesired position in a non-human genome, comprising (a) using bacterialhomologous recombination to precisely engineer a genetic modificationwithin a large cloned genomic fragment to form a large targeting vector,wherein the large targeting vector comprises a human DNA sequence thatis homologous to a non-human DNA sequence of a non-human eukaryoticcell; (b) introducing the large targeting vector into the non-humaneukaryotic cell to modify an endogenous chromosomal locus of interest inthe cell to form a modified allele comprising the human DNA sequence;and, (c) assaying the cell for the modified allele.
 2. The method ofclaim 1, wherein the non-human eukaryotic cell is a mammalian embryonicstem cell.
 3. The method of claim 2, wherein the mammalian embryonicstem cell is a rodent embryonic stem cell.
 4. The method of claim 3,wherein the rodent embryonic stem cell is selected from a mouseembryonic stem cell and a rat embryonic stem cell.
 5. The method ofclaim 1, wherein the large cloned genomic fragment comprises a humangene or gene fragment.
 6. The method of claim 1, wherein the DNAsequence that is homologous to a DNA sequence of the non-humaneukaryotic cell is an orthologous coding sequence.
 7. The method ofclaim 6, wherein the method is employed to replace a murine geneticlocus with an orthologous human genetic locus.
 8. The method of claim 1,wherein the method is employed to insert a new coding region, insert orreplace a gene segment, or insert or replace a regulatory element. 9.The method of claim 1, wherein the method is employed to replace anendogenous locus.
 10. The method of claim 1, wherein the method isemployed to create a conditional allele.
 11. The method of claim 1,wherein the large targeting vector is capable of accommodating DNAfragments greater than 20 kb.
 12. The method of claim 1, wherein thelarge targeting vector is capable of accommodating DNA fragments greaterthan 100 kb.
 13. The method of claim 1, wherein the large targetingvector is greater than 140 kb.
 14. The method of claim 1, wherein thelarge targeting vector is greater than 190 kb.
 15. The method of claim1, wherein the large targeting vector is greater than 235 kb.
 16. Themethod of claim 1, wherein the large targeting vector is greater than256 kb.
 17. The method of claim 1, wherein the large targeting vector isgreater than 265 kb.
 18. The method of claim 1, wherein the largetargeting vector is greater than 288 kb.
 19. The method of claim 1,wherein the large targeting vector is greater than 308 kb.
 20. Themethod of claim 1, wherein the large targeting vector is greater than322 kb.
 21. The method of claim 1, wherein the large targeting vector isgreater than 368 kb.
 22. The method of claim 1, wherein assaying thecell for the modified allele comprises detecting modification of aparental allele.
 23. The method of claim 1, wherein the large targetingvector comprises a sequence selected from a loxP site and a FRT site.24. A method for targeting a large cloned genomic fragment to a desiredposition in a non-human genome, comprising (a) using bacterialhomologous recombination to precisely engineer a genetic modificationwithin a large cloned genomic fragment to form a large targeting vector,wherein the large targeting vector comprises a human DNA sequence thatis orthologous to a non-human DNA sequence of a non-human eukaryoticcell; (b) introducing the large targeting vector into the non-humaneukaryotic cell to modify an endogenous chromosomal locus of interest inthe cell to form a modified allele comprising the human DNA sequence;and, (c) assaying the cell for the modified allele.
 25. The method ofclaim 24, wherein the non-human eukaryotic cell is a mammalian embryonicstem cell.
 26. The method of claim 24, wherein the mammalian embryonicstem cell is a rodent embryonic stem cell.
 27. The method of claim 26,wherein the rodent embryonic stem cell is selected from a mouseembryonic stem cell and a rat embryonic stem cell.
 28. The method ofclaim 24, wherein the large cloned genomic fragment comprises a humangene or gene fragment or a mouse gene or gene fragment.
 29. The methodof claim 24, wherein the orthologous human DNA sequence is a gene codingsequence.
 30. The method of claim 27, wherein the method is employed toreplace a murine genetic locus with an orthologous human genetic locus.31. The method of claim 24, wherein the method is employed to insert anew coding region, insert or replace a gene segment, or insert orreplace a regulatory element.
 32. The method of claim 24, wherein themethod is employed to replace an endogenous locus.
 33. The method ofclaim 24, wherein the method is employed to create a conditional allele.34. The method of claim 24, wherein the large targeting vector iscapable of accommodating DNA fragments greater than 20 kb.
 35. Themethod of claim 24, wherein the large targeting vector is capable ofaccommodating DNA fragments greater than 100 kb.
 36. The method of claim24, wherein the large targeting vector is greater than 140 kb.
 37. Themethod of claim 24, wherein the large targeting vector is greater than190 kb.
 38. The method of claim 24, wherein the large targeting vectoris greater than 235 kb.
 39. The method of claim 24, wherein the largetargeting vector is greater than 256 kb.
 40. The method of claim 24,wherein the large targeting vector is greater than 265 kb.
 41. Themethod of claim 24, wherein the large targeting vector is greater than288 kb.
 42. The method of claim 24, wherein the large targeting vectoris greater than 308 kb.
 43. The method of claim 24, wherein the largetargeting vector is greater than 322 kb.
 44. The method of claim 24,wherein the large targeting vector is greater than 368 kb.
 45. Themethod of claim 24, wherein assaying the cell for the modified allelecomprises detecting modification of a parental allele.
 46. The method ofclaim 24, wherein the large targeting vector comprises a sequenceselected from a loxP site and a FRT site.
 47. A method for modifying adesired position in a non-human genome, comprising (a) using bacterialhomologous recombination to precisely engineer a genetic modificationwithin a large cloned genomic fragment to form a large targeting vector,(b) introducing the large targeting vector into the non-human eukaryoticcell to modify an endogenous chromosomal locus of interest in the cellto form a modified allele comprising the large targeting vector; and,(c) assaying the cell for the modified allele.
 48. The method of claim47, wherein the modification is a deletion of a coding sequence, genesegment or regulatory element.
 49. The method of claim 47, wherein themodification is an alteration of a coding sequence, gene segment orregulatory element.
 50. The method of claim 49, wherein the alterationcomprises substitution of an endogenous gene or locus with anorthologous gene or locus.
 51. The method of claim 47, wherein themodification comprises an insertion of a coding region, gene segment orregulatory element.
 52. The method of claim 51, wherein the modificationcomprises an insertion of a reporter gene into an endogenous gene,wherein the reporter gene is under transcriptional control of theendogenous gene.
 53. The method of claim 52, wherein the reporter geneis β-galactosidase (LacZ).
 54. The method of claim 47, wherein themodification comprises the creation of a conditional allele.
 55. Themethod of claim 47, wherein the modification comprises the introductionof loxP and/or FRT sites.
 56. The method of claim 47, wherein themodification comprises the replacement of an endogenous coding sequenceor gene segment with an orthologous coding sequence or gene segment.